A known catalyst device carries silver oxide by use of alumina as a carrier coat material (for example, refer to Japanese Patent No. 2801423). Such a silver-alumina-based catalyst device adsorbs NO2 in the exhaust gas and releases the adsorbed NO2 when a temperature thereof becomes a set temperature. When the temperature of the silver-alumina-based catalyst device becomes the set temperature, a NOx catalyst device arranged downstream thereof has been activated and thus can purify to reduce the NO2 which flows out from the silver-alumina-based catalyst device.